Note: Descriptions are shown in the official language in which they were submitted.
UK9-87-006
DISPLAY SYSTEM WITH SYMBOL FONT MEMORY
Description
The present invention relates to a display system in which symbols
are generated on a screen by copying blocks of pixels representing the
symbols from general purpose storage into a display buffer.
Display systems in which symbols are generated on a screen in this
way may take many different forms. One possible form is that of a
workstation such as a personal computer. It is common for the display
of a workstation to be a raster scanned display device such as a
cathode ray tube device tCRT~ which requires a refresh (or display)
buffer. In many prior art systems the display buffer is in practice a
part of the workstation's random access memory ~RAM) in the sense that
i~ is within the address space of the workstation processor. This is
not always the case though. Either way, it is usual to configure the
display buffer as an all points addressable (APA) buffer (ie. each item
of pel data can be se~arately accessed) in order that a sufficiently
high data bandwidth can be achieved in order to support the display
while still allowing the buffer to be updated.
Whether the display buffer is part of the workstation RAM or is
physically separate, the relatively high cost of configuring storage as
an APA buffer compared to general purpose RAM or ROM means that it is
desirable to ~eep the display buffer as small as possible.
A low cost prior approach to the problem of how to add symbols to
the display when updating it is that adopted in standard personal
computers with a colour graphics adapter (CGA). In this sort of
computer a symbol table containing each of the symbol definitions for a
set of symbols is stored in general purpose RO~I, and each time a symbol
is selected for display on the screen in APA mode, the block of pels
forming the definition for that symbol is copied from the symbol table
into an APA display buffer. This approach has the disadvantage,
however, that it is relatively slow.
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UK9-87-006 2
An alternative, prior, approach which provides increased
performance at increased cost is to provide a APA display buffer
which includes both on-screen APA storage for the pel data
forming a screen of data for display and off-screen APA storage
for a symbol table with the symbol definitions for each of the
symbols of a symbol set. Special purpose hardware is provided
for copying the symbol definitions from the off-screen to the
on-screen parts of the display buffer. This approach, which was
adopted, for example, in the IBM (Registered Trade Mark)
3270PC/GX and IBM 3270 Personal Computer AT*/GX personal
computers has the disadvantage that it requires the APA display
buffer to be larger, and is consequently more expensive than that
in a standard personal computer. Although this approach is more
efficient than the former one, it is not as efficient in many
applications as might at first be expected. The reason for this
is that in applications such as document preparation in general
and desk-top publishing in particular, it is necessary to provide
a number of different symbol sets (eg. different character
fonts). As it is not viable to provide a display buffer which is
large enough to accommodate all the symbol fonts at one ti~e, the
compromise was adopted to provide storage in the APA display
buffer for one complete storage font and to load the symbol table
required for a selected font from general purpose RAM each time a
font change is required. This process is time consumlng and
presents a significant system overhead when mflny font changes are
required,
A slmilar approach to the la5t menticned one has been
adopted in per80nal computers including so-called enhanced
graphics adapters (EGAs) in that a selected symbol font is
transferred into an off-screen portion of an APA display buffer,
although the details of the management of the buffer is
different. Rather than copying symbol definitions from the
off-screen to the on-screen portions of the buffer, pointers to
the definitions are instead stored in the on-screen portion and
data from the on-screen and the off-screen portions are then
merged in order to refresh the display. In the same way as in
the last mentioned approach, the copying of a symbQl table every
time a font change is required is time consuming and presents a
significant system overhead when many font changes are required.
*Trade Mark
1~94381
UK9-87-006 3
The disadvantages of the prior art are overcome by a display
system in accordance with the present invention, in which are provided
a symbol table in general purpose storage which includes blocks of data
forming symbol definitions for each of a set of symbols form~ng a
desired symbol set, an all points addressable (APA) display buffer
which includes on-screen APA storage for data indicative of pels to be
displayed on a screen and off-screen APA storage for symbol defini-
tions, and buffer control logic for associating symbol definitions in
off-screen APA storage with locations in on-screen APA storage such
that symbols are displayed at desired locations on the display screen,
wherein the display system additionally comprises cache control logic
for copying the definition of a symbol for display from the symbol
table in general pUrpose Storage to off-screen APA storage if the
definition for the selected symbol is not already held in off-screen
APA storage, whereby the off-screen APA storage is configured as a
cache for holding definitions for selected symbols.
A display system in accordance with the present invention enables
increased performance to be achieved at reasonable cost in that the
advantages of special purpose buffer control logic can be exploited
without large off-screen APA storage requirements and heavy processing
overheads.
In a particular embodiment of a display system in accordance with
the invention there is additionally provided in general purpose storage
a cache table comprising an entry for each symbol in the symbol set for
pointing to the location in off-screen APA storage, if any, at which
the corresponding symbol definition is held, each entry in the cache
table also being associated with the location in the symbol table at
Which the corre5ponding symbol definition is stored.
As a plurality of display fonts are provided in this particular
embodiment, a plurality of symbol tables for different symbol sets are
stored in general purpose storage and the display system is provided
with pointer means under the control of the cache control logic for
associating the cache table with the symbol table for the desired
symbol set.
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UK9-87-006 4
In this particular embodiment, the cache control logic can be
arranged to interrogate the cache table when a selected symbol is to be
added to the display in order to identify the location, if any, at
which the definition for the selected symbol is held in off-screen ~PA
storage, to cause the buffer control logic to associate the selected
symbol definition in off-screen APA storage with the on-screen APA
storage such that the symbol is displayed at a desired location on the
display screen, otherwise to copy the definition for the selected
symool from the appropriate symbol table entry in ~eneral purpose
storage to a free location in off-screen APA storage, to store an
indication in the appropriate entry in the cache table of the location
in off-screen APA storage at which the definition for that sy~bol is
now held and to cause the buffer control logic to associate the select-
ed definition in off-screen APA storage with the on-screen APA storage
such that the symbol is displayed at the desired location on the
display screen
In order to enable a more complete understanding of the present
invention, there follows a description of three prior art display
systems and of two specific display systems in accordance with the
present invention, with reference to the accompanying drawings in
which:
Figure 1 is a schematic diagram of aspects of a first prior art
display system;
Figure 2 is a schematic diagram of aspects of a second prior art
display system;
Figure ~ is a schematic diagram of aspects of a third prior art
display system;
Figure 4 is a schematic diagram of aspects of a first display
system in accordance with the present invention;
Figure S is a schematic diagram of aspects of a second display
system in accordance with the present invention;
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UKs-87-006 5
Figure 6 is a flow diagram presenting an overview of the operation
of the display systems of Figures 4 and 5; and
Figure 7 is a block diagram of a specific implementation of the
display adapters of Figures 4 and 5 in the form of a workstation.
Figure l is a schematic block diagram showing certain aspects of a
prior art display system. The display system in question is a conven-
tional personal computer including a colour graphics adapter (CGA). As
the structure of such computers are well known in the art, only those
aspects which are relevant to the present description have been shown.
A symbol table 20, 20', 20", etc is stored in the general purpose
random access memory (RAM) 22 of the computer for each of a number of
symbol sets (eg. different symbol fonts). Each symbol table (eg. 20)
contains a definition of each of the 256 symbols in the set in terms of
a pel (or pixel) map of the symbol needed to generate the symbol on a
display screen ~not shown). In order to select the symbol table which
is currently in use, a pointer 24 is stored in the general purpose RAM
as well. A display buffer ~0 is provided for containing the pel
information which is used to generate an image on the display device of
the personal computer. This buffer is provided as part of the rar,dom
access memory in the sense that it is within the address space of the
personal computer's processor, but it is configured as an APA buffer.
The data stored in the display buffer is read out in synchronism with
the scanning of the display screen. In this Figure, and also Figures 2
to 5 solid arrows have been used to indicate data transfer paths and
arrows with dashed lines have been used to indicate pointing or addres-
sing paths.
In operation, each time a symbol set iS selecte~, the pointer 24
is set to point to the bottom 201 of the appropriate symbol table 20.
Then, when a symbol string 26 is to be added to the display screen at
locations 321, 322, 323, the block of pels forming the definition for
each of the symbols a,p,a in the string is copied from the appropriate
symbol table into the display buffer. Each symbol in the symbol stream
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UK9-87-006 6
is represented by a code 261, 262, 263, which, in combination
with the pointer is used to address the appropriate location for
that symbol in the selected symbol table. This approach has the
disadvantage, however, that it is relatively slow as the
bandwidth of the path from the symbol table to the display buffer
is limited.
Figure 2 is a schematic block diagram of a second prior art
display system. This display system is a personal computer such
as an IBM 3270PC/GX or an IBM 3270 Personal Computer AT/GX which
is provided with a more sophisticated graphics adapter. As the
general structure of such computers are well known in the art,
only those aspects which are relevant to the present description
have been shown. In this computer, a symbol table 20, 20', 20",
etc. is stored in the general purpose random access memory (RAM)
22 for each of a number of symbol sets ln the same way as in the
Figure 1 system. Once agaln each symbol table (eg. 20) contains
a definition of each of 256 symbols in the set in terms of a pel
map of the symbol as it would appear when displayed on a display
screen ~not shown). A display buffer 30 is also provided as part
oE the random access memory in the sense that it is within the
address space of the personal computer's processor, but it is
configured as an APA buffer. In this prior system however, the
display buffer includes both on-screen APA storage 32 for the pel
data forming a screen of data for display and additionally
off-screen APA storage 34 for a symbol table 21 with the symbol
deflnitions for each of the symbols of a symbol set. Moreover,
special purpose block transfer hardware 36 is provided for
copying the symbol definitions from the off-screen to the
on-screen parts of the display buffer.
In operation, each time a symbol set is selected, the full
symbol table for that desired font is copied from the general
purpose RAM to the off-screen storage part of the display buffer.
A pointer 38, which is held in general purpose RAM, is set to
point to the location 211 at which the selected font table starts
in the off-screen storage part of the display buffer. Ther., when
a symbol string 26 is to be added to the display screen at
locations 321, 322, 323 the block transfer logic
129438~
UK9-87-006 7
copies the block of pels forming the definition for each of the symbols
a, p, a in the string from the off-screen to the on-screen part of the
display buffer. Each symbol in the symbol stream is represente~ by a
code 261, 262, 263 which, in combination with the pointer 38 is used to
address the appropriate location for that symbol in the symbol table
held in off-screen APA storage. The bandwidth provided for the
transfer of symbol definitions between the off-screen and the on-screen
parts Of the APA display buffer by the special pUrpose hardware iS high
so that the display Of individual symbols iS Very rapid. However~
because of the the limited bandwidth between the general purpose RAM
and the off-screen part of the display buffer, the copying of the
symbol table each time a font change occurs is very time consuming.
This presents a significant system overhead when many font changes are
required, for example in desk top publishing applications and other
document preparation applications.
Figure 3 is a schematic block diagram of a third prior art display
system. This display system is a personal computer provided with an
Enhanced Graphics Adapter or EGA. This computer is generally similar
to the prior art display system sho~n in Figure 2. A different app-
roach to the organisation of the APA display buffer is, however,
adopted. The display buffer includes both on-screen APA storage 32
indicative of pels to be displayed on the display screen and off-screen
APA storage 34 for a symbol tabIe 21 with the symbol definitions for
each of the symbols of a symbol set. The on-screen APA storage does
not contain the actual pel information for the symbols to be displayed,
but rather contains pointers a', p', a' to the symbol definitions in
the off-screen APA storage. Special purpose merge control hardware 37
is provided for merging the data output from the on-screen and the
off-screen APA parts of the display buffer to form the data stream for
generating the display.
The operation of this system can be summarised as follows. Each
time a symbol set, or font is selected, the full syl~ol table for that
desired set, or font is copied from the general purpose RAM to the
~,z94381
UK9-87-006 8
off-screen storage part of the display buffer. A pointer 38, which is
held in general purpose RAM, is set to point to the location 211 at
which the selected symbol table starts in the off-screen storage part
of the display buffer. Then, when a symbol string 26 is to be added to
the display screen at locations 321, 322, 323 pointers a', p', a' are
stored at appropriate locations in the on-screen part of the display
buffer using the codes 261, 262, 263 for the respective symbols in
combination with the pointer 38. When the display is refreshed, the
pointers in the on-screen part of the APA display buffer are used by
the merge control logic to select the symbol definitions for display
from the off-screen part of the APA display buffer. This display
system suffers from the same problems as the Figure 2 system because of
the the limited bandwidth between the general purpose RAM and the
off-screen part of the display buffer which means that the copying of
the full symbol table each time a font change occurs is very time
consuming.
The present invention does not suffer from the disadvantages of
the prior art. Figure 4 is a schematic block diagram of aspects of a
particular embodiment of a display system in accordance with the
present invention. The display system comprises a symool table 20,
20', 20", etc for each of a number of symbol sets. The symbol sets can
be, for example, different character sets or fonts. The symhol tables
are stored in the general purpose memory of the display system.
Normally the symbol tables are stored in general purpose random access
memory. They could, however, be stored in another general purpose
memory such as a read only memory. The basic principle is that the
storage for the symbol sets is comparatively inexpensive and plentiful.
As in the prior art systems, each symbol table (eg. 20) contains a
definition of each of the symbols in the set in terms of a block of
pels forming a pel map of the symbol as it would appear when displayed
on a display screen (not shown). Each set will normally comprise 256
symbols, although the sets can be larger or smaller as required. In
order to indicate which symbol table is currently in use, a pointer 24
is provided. An APA display buffer 30 is provided which includes both
l.Z943Rl
UK9-87-006 9
on-~creen APA storage 32 for data indicative pels to be displayed on a
screen and off-screen APA storage 34 for a symbol table with the symbol
definitions ~or each o~ the symbols of a symbol set. Also, in the same
way as in the prior system illustrated in Figure 2, special purpose
hardware 36 is provided for copying the symbol definitions from the
off-screen to the on-screen parts of the display buffer. In addition
to the elements mentioned above, the display system illustrated in
Figure 4 also comprises a cache table 40 and cache control logic 42.
The cache control logic is used to configure the off-screen APA stor-
age, or part thereof, as a cache for the definitions of in~ividual
symbols that have been selected for display. The cache table comprises
an entry ~eg. 2 bytes long) for each symbol in a selected symbol set
for pointing to the location in said off-screen storage, if any, at
which the corresponding symbol definition is stored.
The operation of the display system shown in Fi~ure 4 will be
explained with refere~ce to the flow diagram in Figure 6. As the
stream of symbols to be added to the display is received, a check is
made to see if a symbol set, or font change is required (block 9o). If
a new s~m~ol font is selected ~block 91), the cache control logic
resets the cache table and the associates the cache table with the
symbol table for the selected ~ont by setting the pointer 24 to point
to the base 201 of that symbol table. The cache-table has the same
number o~ entries (eg. 256) as ~e symbol tables whereby each entry in
the cache table can be associated with a particular symbol definition
in the table for the selected font. The effect of resetting the cache
table is that symbol definitions can only be accessed from the symbol
table which is identified by the pointer. Any symbol definitions which
were previously stored in the off-screen area of the display buffer
need not be erased as such, they just cannot be accessed because the
pointers in the cache table have been erased.
Figure 4 sho~s the first three symbols a, p, a in a symbol stream
26 which is to be displayed on the display screen. Each symbol in the
stream is represented by a code 261, 262, 263 which is used to address
the cache table.
129~313~
U~9-87-006 10
Por each successive symbol in the stream, the cache control logic
interrogates the entry in the cache table identified by the code repre-
senting eaeh said symbol (block 92).
If the particular symbol represented by the received code has been
used since the eache table was reset, there will be a pointer (a', p')
to an off-screen display buffer location stored in the appropriate
location in the cache table and the control logic will know that the
block of pel data forming the definition for that symbol is to be found
in the off-screen display buffer location pointed to by the content of
the appropriate location in the cache table. In this case the control
logie causes (block 96) the block transfer logie to copy the block of
pel data which was previously stored in the location in off-screen
storage pointed to into the on-sereen storage.
If, on the other hand, the partieular symbol represented by the
received eode has not been used sinee the eaehe table was reset, there
will be no pointer (a', p') in the appropriate entry in the cache table
pointing to the off-screen display buffer and the control logic will
know that the definition for that symbol is to be found in the corres-
ponding location in the currently operative symbol table 20 as identi-
fied by the pointer 24. In this ease, the eaehe control logic tests
(block 93) whether the cache is full. If the caehe is full, the
eontrol logie resets the caehe table tbloek 94). The effect of reset-
ting the eaehe table is that the first time any particular symbol is to
be added to the display following the resetting operation, the eorres-
ponding definition ean only be aeeessed from the symbol table whieh is
identified by the pointer. Conse~uently, the symbol definitions which
were previously stored in the off-screen area of the display buffer
need not be erased as such.
If the cache table was not full, or following a resetting opera-
tion, the cache control logic then causes (block 95) the block of pel
data forming the definition for the selected symbol to be copied from
the appropriate entry in the symbol table identified by the pointer 24
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UK9-87-006 11
to a free location 341, 342 the off-screen storage and causes a pointer
to this free location to be stored in the appropriate entry in the
cache table. The cache control logic then causes (block 96) the block
transfer logic 36 to copy the block of pel data which has just been
stored in the off-screen storage into the on-screen storage.
If there is a further symbol to be processed (block 97) the cache
control logic repeats the steps mentioned above.
Figure 5 is a schematic block diagram of a second display system
in accordance with the invention. This display system is generally
similar to the system shown in Figure 4, but with special purpose merge
control logic for the APA display buffer along the lines of that in the
Figure 3 prior art system rather than block transfer logic. The
on-screen APA storage does not contain the actual pel information for
the symbols to be displayed, but rather contains pointers a', p', a' to
blocks of pel data forming symbol definitions at 341, 342 in the
off-screen APA storage. Vnlike the Figure 3 prior art system, however,
a complete symbol table is not copied from general purpose storage to
the off-screen part of the display buffer. Instead the off-screen APA
storage is configured as a cache by cache control logic 42.
The cache control logic 42' in this embodiment operates as des-
cribed above for the Figure 4 e~bodiment of the invention, with excep-
tion of the operations to be performed in block 96 in Figure 6. The
step of updating the buffer in this embodiment comprises the cache
control logic causing a pointer 24 to the symbol definition in off-
screen APA storage to be written into on-screen APA storage such that,
on subsequent scans of the on-screen APA storage, the merge logic
causes the symbol to be reproduced at the appropriate location on the
display. The pointer a', p', to be inserted in a particular on-screen
location is the same as the pointer in the cache table for the symbol
in question.
` 12943~1
UK9-87-006 12
Figure 6 is a schematic diagram of a specific implementation of a
display system in accordance with the invention in which the system is
in the form of a workstation comprising a number of different system
units connected via a system bus 43. The system bus comprises a data
bus 44, an adaress bus 46 and a control bus 48. Connected to the
system bus is a microprocessor 50, random access memory 52, a keyboard
adapter 54, a display adapter 56, an I/0 adapter 58 and a communica-
tions adapter 60. The keyboard adapter is used to connect a keyboard
62 to the system bus. The display adapter connects the system bus to a
display device 64. The l/0 adapter likewise provides a connection
between other input/output devices 66 teg. DASDs) and the system bus,
and the communications adapter allows the workstation to be connected
to and to communicate with an external processor or processors such as
a host processor (not shown).
The display adapter is preferably implemented as special purpose
hard-wired logic to take advantage of the speed of such logic. It
could however be implemented with a high speed microprocessor and
storage elements and appropriate code.
The display adapter includes an APA display buffer 30 which is not
addressed directly by the processor 50. The display device can,
however access the display buffer in order to fetch the data correspon-
ding to the individual picture e~ements on the screen. The data are
fetched in synchronism with the scanning of the display screen. To
facilitate this the information in the display buffer is organised in
accordance with the scanning sequence of the display refresh circuitry.
The display buffer comprises an on-screen APA storage area and an
off-screen APA storage area, as shown in Figure 4 or Figure 5. The
on-screen storage area contains data indicative of pels to be displayed
on the display device's screen and the off-screen storage is configured
as a cache for containing the symbol definitions of symbols selected
for display. The display adapter comprises hard-wired logic buffer
control logic.
~ ,9~381
~K9-87-006 t3
For a Figure 4 type embodiment of the invention, the on-screen
.is for storing ~ctual pel datfl for disp].ay and the buffer control
logic is block transfer logic for copying blocks oE dflta forming
symbol definitions for selected symbols from the off-screen area to
the on-screen are~ of the display bu:ffer. For A Figure 5 type
embodi.ment of the invention, the on-screen is for storlng, inter
alia, pointers -to symbol defi.niti.ons :Ln the of~-screen Storflge area
and the buffer control logic is merge control logic for merglng data
from the off-screen and the on-screen areas oi-' the displAy buffer in
order to re:Eresh the display screen.
~ he cache control loglc 42, 42' is implemented by means of
suitable code which is held in general purpose RAM and ls used to
control the workstatlon processor to cArry out the Eunctions of that
logic. The csche table, the symbol tables and the pointer are all
configured i.n the workstati.on' 9 genernl purpose RAM 52. Of course,
the logic could equal.l.y be prov:lded in the form of speci~l purpose
hflrd-wired logic, with one or more of th~ cflche tflblr,~ the symbol
tabl.es or the pointer provi.ded i.n the form of spec:iAl regi.sters.
~ deta.LIed descr:lpt:i.on of the actufll format itl wh.Lcil the data i.9
st:ored hAS not been prov:i.de(l a~ thl~ cfln tAke Any one Of a number Or
convent.ional forms. Ilowever, .ill order that the d:l~play may be scsnne(l
~t a sllfficl.erltly h.igh rate wi.thollt resort.ing to expens.ive solut:Lons,
the im~ge dat~ for display .is preferably stored in the display buffer
in accordance wLth the scanning cycle :for the display.
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UK9-87-006 14
Tt will be appreciated that many modifications and/or addltions
to th~ p~rt;cularly described embodiments are possible withln the
scope of the appended clsims.
For example, as the cache control logic is implemented in
software in tlle specific embodiments of the invention described
flbove, a simple cache mflnflgement algorithm, namely "clear when full"
i..q used. Clearly, howev~r, a more compl.icated cflche mana~ement
algori.thm quch as "least recent1.y used" could be employed,
partl.cu].arly if the cflche control logi.c were tmplemented ln speci.fll
purpose logit:. In prflctice the simple algorithm hfls proved to be
ef:Fective bccause, on flVerage in normal us~ge, only about 64 oE the
avallable 25fi symbols in a symbol set flre used between font changes.
With a cache whlch can accommodate, for exsmple, 128 symbol
defl.nitions the cache rarely becomes futl therefore.
Also, fl1thou~h the preqent descript::Lon ha~q been part:l.cular1.y
concerned w.ith display systems :Ln the form oE workstat.ions, i.t will.
be appreciflted that the preqent inventl.on ls fllso appl:Lcab1.e to
display systems of th~ host-tQrmlnal type where. the symbol tables are
stored in the host and wh~re there t~q only t:imited storage in the
terminal for symbol defln1tions. Tn th.ts sort o flrrangement the
cache control. :logl.c flnd the cache tnble would be 1.ocated :Ln the host
along with the symbol tables for the symbol deflnitions. The cache
control loglc would ensure that only the symbol definitions which are
required for dlsplay are down loaded into the term.1.nal in the same
mflnner as deqcr.tbed above for a workstatlon.
